Pre- and post-junctional actions of procaterol, a beta 2-adrenoceptor stimulant, on dog tracheal tissue

Traditional oriental herbal medicine, Bakumondo-to, suppresses vagal neuro-effector transmission in guinea pig trachea

OBJECTIVE

Bakumondo-to (Maimendong tang) is a traditional oriental herbal medicine that has been used as an antitussive agent. We previously demonstrated that Bakumondo-to attenuates airway hyperresponsiveness induced by ozone. However, the mechanism(s) responsible for this effect remains unclear. In the present study, we examined the mechanism whereby Bakumondo-to inhibits ozone-induced airway hyperresponsiveness. First, we examined the effect of Bakumondo-to on prostanoids production, which are key mediators to airway hyperresponsiveness after ozone exposure. Second, we studied its effects on the vagal neuroeffector transmission, because vagal nerve is likely to play an important role in airway hyperresponsiveness after ozone.

METHODS

We measured the effects of Bakumondo-to on the concentrations of prostanoids in bronchoalveolar lavage fluid before and after ozone. We evaluated the effects of Bakumondo-to on the contraction of guinea pig tracheal smooth muscle evoked by electrical field stimulation (EFS) or the exogenous application of acetylcholine (ACh). Isometric tension of tracheal strips was measured in the presence of indomethacin (10(-6) M) and of guanethidine (10(-6) M).

RESULTS

Ozone caused significant increase in prostaglandin E2 (PGE2) and thromboxane B2 (TXB2); however, Bakumondo-to did not affect the increase in these prostanoids. Bakumondo-to (0.01 mg/mL-1 mg/mL) significantly suppressed the contraction evoked by EFS, but did not affect the ACh-evoked contraction, indicating that Bakumondo-to suppressed tracheal smooth muscle contraction pre-junctionally.

CONCLUSION

These results suggest that the mechanism by which Bakumondo-to inhibits airway hyperresponsiveness depends on inhibiting the release of acetylcholine from vagal nerve terminals.

Modulation of cholinergic responsiveness through the [beta]-adrenoceptor signal transmission pathway in bovine trachealis

The effects of pharmacological stimulation at different levels of the beta-adrenoceptor (AR) pathway, including the receptor, the receptor-coupled Gs protein, and adenylyl cyclase, were studied by simultaneous measurements of acetylcholine (ACh) release and isometric force evoked by electric stimulation in isolated bovine trachealis. The beta-AR agonists isoproterenol (10-6 and 10-5 M) and salbutamol (10-7 to 10-5 M) significantly attenuated both ACh release and contractile force. Forskolin, at 10-6 M, significantly increased ACh release without effect on contractile force, whereas at 10-5 M it increased ACh release but significantly decreased force. Activation of Gs protein by cholera toxin (10 microg/ml) significantly attenuated both ACh release and contractile force, but its effect on ACh release was abolished by calcium-activated potassium (KCa)-channel blocker iberiotoxin (10-7 M). The KCa-channel opener NS-1619 (10-4 M) attenuated significantly both ACh release and contractile force. It is concluded that beta-AR agonists attenuate cholinergic neurotransmission in isolated bovine trachealis model by a mechanism not involving cAMP but KCa channels.

Ionic mechanisms and Ca(2+) regulation in airway smooth muscle contraction: do the data contradict dogma?

In general, excitation-contraction coupling in muscle is dependent on membrane depolarization and hyperpolarization to regulate the opening of voltage-dependent Ca(2+) channels and, thereby, influence intracellular Ca(2+) concentration ([Ca(2+)](i)). Thus Ca(2+) channel blockers and K(+) channel openers are important tools in the arsenals against hypertension, stroke, and myocardial infarction, etc. Airway smooth muscle (ASM) also exhibits robust Ca(2+), K(+), and Cl(-) currents, and there are elaborate signaling pathways that regulate them. It is easy, then, to presume that these also play a central role in contraction/relaxation of ASM. However, several lines of evidence speak to the contrary. Also, too many researchers in the ASM field view the sarcoplasmic reticulum as being centrally located and displacing its contents uniformly throughout the cell, and they have focused almost exclusively on the initial single [Ca(2+)] spike evoked by excitatory agonists. Several recent studies have revealed complex spatial and temporal heterogeneity in [Ca(2+)](i), the significance of which is only just beginning to be appreciated. In this review, we will compare what is known about ion channels in ASM with what is believed to be their roles in ASM physiology. Also, we will examine some novel ionic mechanisms in the context of Ca(2+) handling and excitation-contraction coupling in ASM.

The prostaglandin E series modulates high-voltage-activated calcium channels probably through the EP3 receptor in rat paratracheal ganglia

The modulation of high-voltage-activated (HVA) Ca2+ channels by the prostaglandin E series (PGE1 and PGE2) was studied in the paratracheal ganglion cells. Prostaglandin E1, E2, STA2 (a stable analogue of thromboxane A2), 17-phenyl-trinor-PGE2 (an EP1-selective agonist) and sulprostone (an EP3-selective agonist) inhibited the HVA Ca2+ current (HVA ICa) dose-dependently, and the rank order of potency to inhibit HVA Ca2+ channels was sulprostone>PGE2, PGE1>STA2>>17-phenyl-trinor-PGE2. SC-51089 (10(-5) M), a selective EP1-receptor antagonist, showed no effect on the PGE1- or PGE2-induced inhibition of the HVA ICa, thereby indicating that PGE1- and PGE2-induced inhibition of the HVA Ca2+ channels is possibly mediated by the EP3 receptor. The PGE1-sensitive component of the current was markedly reduced in the presence of omega-conotoxin-GVIA (3x10(-6) M), but not with nifedipine (3x10(-6) M). PGE1 and PGE2 also inhibited the remaining ICa in a saturating concentration of nifedipine, omega-conotoxin-GVIA and omega-conotoxin-MVIIC, suggesting that R-type Ca2+ channels are involved. The inhibitory effect of PGE1 or sulprostone was prevented by pretreatment with pertussis toxin [islet activating protein (IAP)] or phorbol-12-myristate-13-acetate (PMA), and the protein kinase C (PKC) inhibitor chelerythrine blocked the action of PMA. It was concluded that PGE1 selectively reduces both N- and R-type Ca2+ currents by activating a G-protein probably through the EP3 receptor in paratracheal ganglion cells.

Effects of epinastine hydrochloride on cholinergic neuro-effector transmission in canine tracheal smooth muscle

We determined the effects of epinastine hydrochloride, an anti-asthmatic drug, on cholinergic neuro-effector transmission in canine trachea. Isometric tension of tracheal strips was measured in the presence of indomethacin and propranolol. Epinastine (10(-6) M) significantly suppressed the contraction evoked by electrical field stimulation, but had no effect on the acetylcholine-evoked contraction. An L-type Ca2+ channel blocker, nicardipine, did not suppress the electrical field stimulation-induced smooth muscle contraction and did not alter the inhibitory effect of epinastine. An N-type Ca2+ channel blocker, omega-conotoxin, suppressed the electrical field stimulation-induced contraction in a dose-dependent manner, and in a subthreshold/intermediate concentration abolished the inhibitory effect of epinastine. These findings indicate that epinastine exerts prejunctional inhibitory effects on airway smooth muscle of dogs, presumably by inhibiting acetylcholine release from vagal nerve terminals, and suggest that this effect is mediated by N-type Ca2+ channels.

Potentiation of the relaxing action of isoproterenol by forskolin in rabbit aortic rings: the involvement of beta 2-adrenoceptors

1. Forskolin, an activator of adenylate cyclase, potentiated the relaxing response to isoproterenol in rabbit aortic rings precontracted by phenylephrine (PE). 2. The potentiating effect of forskolin was inhibited by propranolol, a beta-adrenoceptor inhibitor, but not by methylene blue, a guanylate cyclase inhibitor. 3. The relaxing response to terbutaline, a beta 2-adrenoceptor agonist, but not lower concentrations of dobutamine, a beta 1-adrenoceptor agonist, also was potentiated by forskolin. Forskolin, however, potentiated the relaxing response to high concentrations of dobutamine, which activates both beta 1- and beta 2-adrenoceptors. 4. Yohimbine, an alpha 2-adrenoceptor inhibitor, glyburide, an ATP-sensitive K+ channel inhibitor, iberiotoxin, a Ca(2+)-activated K+ channel inhibitor, or endothelium-removal failed to affect the potentiating effect of forskolin. 5. Dibutyryl cyclic AMP (cAmp) also potentiated the relaxing response to terbutaline. 6. These results suggest that in rabbit aortic rings forskolin causes the apparent potentiation of isoproterenol-induced relaxation by mainly affecting the relaxing response due to the activation of beta 2-adrenoceptors by the forskolin-induced increase in the level of cAMP.

The inhibitory effects of iberiotoxin and 4-aminopyridine on the relaxation induced by beta 1- and beta 2-adrenoceptor activation in rat aortic rings

1. In rat aortic rings contracted by phenylephrine, the relaxation induced by isoprenaline was partly inhibited by iberiotoxin, (ibTX), tetraethylammonium, 4-aminopyridine (4-AP) and 1,9-dideoxyforskolin, but not by glibenclamide. 2. In the presence of 4-AP, 1,9-dideoxyforskolin failed to inhibit further the relaxant response to isoprenaline. Cromakalim-induced relaxation was inhibited by glibenclamide. 3. In the absence of endothelium, ibTX and 4-AP still inhibited the relaxant response to isoprenaline. 4. The inhibitory effect of ibTX on the relaxant response to isoprenaline was eliminated by pretreatment with ICI-118,551, a beta 2-adrenoceptor antagonist, but not by atenolol, a beta 1-adrenoceptor antagonist. 5. The inhibitory effect of 4-AP on the relaxation induced by isoprenaline was abolished by atenolol, but not by ICI-118,551. 6. The inhibitory effect of ibTX on the isoprenaline-induced relaxation in the presence of atenolol was completely abolished by MDL 12,330A, an adenylate cyclase inhibitor. Further, the inhibitory effect of 4-AP on the isoprenaline-induced relaxation in the presence of ICI-118,551 was markedly reduced by MDL 12,330A. 7. The relaxation induced by dibutyryl cyclic AMP was partly inhibited by 4-AP but not by ibTX. However, in the presence of KT5720, an inhibitor of cyclic AMP-dependent protein kinase, ibTX failed to inhibit further the relaxation induced by isoprenaline. 8. These results suggest that, in rat aortic rings, KCa channels are involved in the relaxation induced by isoprenaline. In addition, KCa channels are mainly activated by beta 2-adrenoceptors through cyclic AMP-dependent pathways. Further, the inhibition of isoprenaline-relaxation by 4-AP may be related to the activation of beta 1-adrenoceptors and cyclic AMP formation.

Inhibition by endothelin-1 of cholinergic nerve-mediated acetylcholine release and contraction in sheep isolated trachea

1. The relative roles of ETA and ETB receptor activation on cholinergic nerve-mediated contraction and acetylcholine (ACh) release were examined in sheep isolated tracheal smooth muscle. 2. Electrical field stimulation (EFS; 90 V, 0.5 ms duration, 1 Hz, 10 s train) applied to sheep isolated tracheal smooth muscle strips induced monophasic contractile responses that were abolished by either 1 microM tetrodotoxin or 0.1 microM atropine, but were insensitive to 10 microM hexamethonium and 100 microM L-NAME. Thus, EFS-induced contractions resulted from the spasmogenic actions of ACh released from parasympathetic, postganglionic nerves. 3. As expected, sheep isolated tracheal smooth muscle preparations did not contract in response to the ETB receptor-selective agonist, sarafotoxin S6c (0.1-100 nM). However, sarafotoxin S6c caused a concentration-dependent and transient inhibition of EFS-induced contractions. The inhibitory effect induced by a maximally effective concentration of sarafotoxin S6c (10 nM; 72.1 +/- 5.7%, n = 6) was abolished in the presence of the ETB receptor-selective antagonist BQ-788 (1 microM). Contractile responses to exogenously administered ACh (10 nM-0.3 mM) were not inhibited by sarafotoxin S6c (1 or 10 nM; n = 7). 4. In contrast to sarafotoxin S6c, endothelin-1 induced marked contractions in sheep isolated tracheal smooth muscle. These contractions were inhibited by BQ-123, consistent with an ETA receptor-mediated response. In the presence of BQ-123 (3 microM), endothelin-1 produced a concentration-dependent inhibition of EFS-induced contractions (30 nM endothelin-1, 68.9 +/- 10.2% inhibition, n = 5). These responses were inhibited by 1 microM BQ-788, indicative of an ETB receptor-mediated process. Endothelin-1 was about 3 fold less potent than sarafotoxin S6c. 5. EFS (90 V, 0.5 ms duration, 1 Hz, 15 min train) induced the release of endogenous ACh (1.94 +/- 0.28 pmol mg-1 tissue, n = 12), as assayed by h.p.l.c. with electrochemical detection. EFS-induced release of ACh was inhibited to a similar extent by 100 nM endothelin-1 (47 +/- 4%, n = 9) and 10 nM sarafotoxin S6c (46 +/- 9%, n = 3). These effects of endothelin-1 on ACh release were inhibited by 1 microM BQ-788 alone (n = 4), by BQ-788 in the presence of 3 microM BQ-123 (n = 4), but not by 3 microM BQ-123 alone (n = 5). 6. In summary, sheep isolated tracheal smooth muscle contains two anatomically and functionally distinct endothelin receptor populations. ETA receptors located on airway smooth muscle mediate contraction, whereas ETB receptors appear to exist on cholinergic nerves that innervate tracheal smooth muscle cells and mediate inhibition of ACh release. The inhibitory effect of ETB receptor stimulation on cholinergic neurotransmission is in stark contrast to the enhancing effects hitherto described in the airways.

Paradoxical facilitation of acetylcholine release from parasympathetic nerves innervating guinea-pig trachea by isoprenaline

1. Previous studies have provided evidence that activation of beta-adrenoceptors on cholinergic nerve terminals can inhibit neurotransmission in the airways. However, in most cases, this conclusion has been based on indirect evidence obtained from mechanical experiments where changes in airways smooth muscle tone were measured. 2. We have assessed whether modulation of cholinergic neurotransmission by beta-adrenoceptor agonists is due to a pre- or post-junctional action by investigating the effect of isoprenaline on contractile responses evoked by exogenous acetylcholine (ACh) and electrical field stimulation (EFS; 4 Hz, 40 V, 0.5 ms pulse width every 15 s), and on EFS-induced ACh release from cholinergic nerves innervating guinea-pig and human trachea. Furthermore, the subtype of beta-adrenoceptor which modulates neurotransmission and the potential role of cyclic AMP in this response were evaluated. 3. In guinea-pig trachea, isoprenaline (1 nM-1 microM) inhibited the contractile response evoked by exogenous ACh (1 microM) to a similar extent to that evoked by EFS (EC50 = 19.9 and 23 nM, respectively). 4. In epithelium-denuded guinea-pig strips treated with indomethacin (10 microM), isoprenaline significantly enhanced EFS-induced ACh release from cholinergic nerve terminals (by 36% at 0.3 microM). This effect was blocked by propranolol and ICI 118, 551 (each 0.1 microM). In contrast, isoprenaline failed to affect EFS-induced ACh release from parasympathetic nerves innervating human trachea. 5. To evaluate the role of cyclic AMP in the beta-adrenoceptor-induced facilitation of cholinergic neurotransmission, the effects of various cyclic AMP elevating drugs on ACh release were studied. Forskolin (10 microM) significantly augmented (by 17%) EFS-induced ACh release, an effect which was not reproduced by 1,9-dideoxyforskolin (10 microM) which does not activate adenylyl cyclase. Similarly, the cyclic AMP analogue, 8-bromo-cyclic AMP (1 mM) and cholera toxin (1 microgram ml-1) facilitated ACh output by 22 and 47% respectively, whereas prostaglandin E2 (PGE2, 0.1 nM-1 microM) inhibited this response (by 67% at 1 microM). 6. Zardaverine (10 microM), a dual inhibitor of the phosphodiesterase (PDE)3 and PDE4 isoenzyme families, did not affect EFS-induced ACh release and failed to facilitate the actions of either isoprenaline or PGE2. Similarly, neither SK&F 94120 (10 microM) nor rolipram (10 microM), selective inhibitors of PDE3 and PDE4 respectively, significantly affected the release of ACh in response to EFS. 7. The result of this study suggests that isoprenaline facilitates cholinergic neurotransmission in guinea-pig, but not human, trachea by activation of pre-junctional beta 2-adrenoceptors, an effect that may be mediated via activation of the cyclic AMP/cyclic AMP-dependent protein kinase cascade. Furthermore, the data presented herein illustrate the need to undertake direct measurements of neurotransmitter release when examining the effect of agents purported to act pre-junctionally.

Thromboxane A2 mimetic (U-46619) induces hyperresponsiveness of smooth muscle in the canine bronchiole, but not in the trachea

It has been reported that cholinergic agonists induce bronchoconstriction by directly stimulating M3 muscarinic receptors on the surfaces of smooth muscle cells. Although thromboxane A2 (TXA2) has been demonstrated to induce airway hyperresponsiveness to cholinergic agonists in vivo, it does not affect the contractile response of smooth muscle to cholinergic agonists in vitro. To investigate the causes for the discrepancy between the in vivo and in vitro data, we compared the effects exerted by a TXA2 mimetic, U-46619, on the smooth muscle of canine trachea and bronchiole. We measured the contractile response to exogenously applied acetylcholine (ACh) before and after the application of a subthreshold dose of U-46619. The subthreshold dose was determined as that dose which did not induce smooth muscle contraction, this being 10(-9) M in the present study. The contractile responses of tracheal strips to ACh were not affected by the subthreshold dose of U-46619. By contrast, the responses of bronchiolar rings were significantly enhanced by this subthreshold dose. The excitatory effect of U-46619 on the ACh-induced contraction was completely prevented by treatment with a TXA2 antagonist, BAY u3405. These results indicate that TXA2 directly increases the responsiveness of smooth muscle in the bronchiole, and suggest that increases in the responsiveness of small airways may play an important role in the development of the airway hyperresponsiveness induced by TXA2.

Different effects of salmeterol, formoterol and salbutamol on cholinergic responses in the ferret trachea

1. In the present study, the inhibitory effects of the selective beta 2-adrenoceptor agonists, salmeterol, formoterol and salbutamol, have been investigated on contractions of ferret trachea induced both by endogenous and exogenous acetylcholine. The aim of the study was to evaluate quantitative and/or qualitative differences in response which may indicate both pre- and post-junctional sites of action. The non-selective beta-antagonist, sotalol, was used to estimate beta-adrenoceptor involvement. 2. Isometric tension was measured in ferret isolated tracheal strips. The inhibitory effects of the drugs were studied on tonic contractions induced by pre-junctional activation with electrical field stimulation (EFS) (2 Hz, 700 mA) or post-junctional activation with exogenous acetylcholine (ACh) (0.5 microM, about EC80), giving a similar degree of smooth muscle response. 3. Concentration-response experiments were performed with formoterol (0.3 nM-0.3 microM) and salmeterol and salbutamol (10 nM-10 microM). The experiments ended with the addition of sotalol (10 microM). 4. All three beta-agonists inhibited the contractions in a concentration-dependent manner. Salbutamol, formoterol and salmeterol inhibited the EFS-induced contractions by 66(8)%, 105(5)% and 103(8)% (mean(s.e. mean)) respectively. ACh-induced contractions were inhibited by 37(6)%, 72(11)% and 33(8)%. Theophylline (10 nM-3 mM) inhibited the contractions to the same degree. 5. beta-Adrenoceptor blockade by sotalol significantly antagonized the inhibitory effects of salbutamol and formoterol on both EFS- and ACh-induced contractions. The effect of salmeterol on ACh-induced contraction was also significantly antagonized, whereas the inhibition of EFS-induced contraction was virtually unaffected. 6. In conclusion, salbutamol, salmeterol and formoterol produced greater inhibitory effects in preparations contracted by EFS than in preparations contracted by exogenously-added ACh. In the case of formoterol and salbutamol, the effects on both levels are most probably due to beta-adrenoceptor stimulation, whereas for salmeterol the dominant pre-junctional effect is probably not mediated via beta-adrenoceptors. This non-beta-mediated effect could represent an additional relaxant mechanism for salmeterol.

Beta 3-adrenoceptors and airways

beta 3-adrenoceptors have been identified in a variety of tissues from humans and animals: adipose tissue, gastrointestinal smooth muscle, rat skeletal muscle, bovine skeletal muscle, and human and canine heart. In the airways, the investigation of the beta 3-adrenoceptors came from studies with a series of novel selective agonists. Stimulation of the "atypical" beta-adrenoceptor increases the active transport of albumin across the ferret tracheal epithelium and the ciliary beat frequency of canine bronchial epithelium. Furthermore, it has been demonstrated that beta 3-adrenoceptors agonists selectively inhibited nonadrenergic noncholinergic contractions of guinea-pig bronchi induced by electrical field stimulation or capsaicin. The presence of functional beta 3-adrenoceptors in the bronchial smooth muscle is disputed and seems to be species-related. In isolated canine bronchi, selective agonists induced a relaxation whereas they had no or slight effect in isolated human, guinea-pig and sheep bronchi. Likewise in man, a fall in airway resistance measured by plethysmography, was mediated by beta 2-adrenoceptors, but not beta 3-adrenoceptors. To conclude, an "atypical" or beta 3-adrenoceptor-mediated modulation of bronchomotricity exists, nevertheless strong species specific differences have been reported.

Beta-adrenoceptors mediate inhibition of [3H]-acetylcholine release from the isolated rat and guinea-pig trachea: role of the airway mucosa and prostaglandins

1. Rat or guinea pig isolated tracheae were labelled with [3H]-choline to measure evoked tritium outflow, which reflects neuronal release of [3H]-acetylcholine. Tritium outflow was evoked either by electrical stimulation of the extrinsic vagal nerve (rat tracheae) or by 27 mM potassium (guinea pig tracheae). 2. In rat tracheae isoprenaline (0.01, 0.1 microM) inhibited evoked [3H]-acetylcholine release, whereas beta 2-adrenoceptor-selective agonists (fenoterol, formoterol, salbutamol) were ineffective. 3. The inhibitory effect of isoprenaline was abolished under the following conditions: (i) presence of propranolol (1 microM) or of the beta 1-selective antagonist CGP 20712 A (0.1 microM); (ii) removal of the mucosa at the start of the experiments; (iii) blockade of cyclooxygenase activity by 3 microM indomethacin. 4. In rat isolated tracheae prelabelled with [3H]-arachidonic acid, isoprenaline (0.1 microM) but not formoterol (0.01 microM) enhanced the outflow of [3H]-prostaglandins (PGD2, PGE2). This effect was blocked by 0.1 microM CGP 20712 A. 5. In guinea pig tracheae electrical stimulation of the extrinsic vagal nerve did not cause a constant release of [3H]-acetylcholine, but 27 mM potassium elicited a reproducible release of [3H]-acetylcholine. In this species both isoprenaline (0.1 microM) and formoterol (0.01 microM) inhibited evoked [3H]-acetylcholine release. Inhibition was abolished under the following conditions: (i) presence of propranolol (1 microM) or of the beta 2-selective antagonist ICI 118551 (0.3 microM); (ii) removal of the mucosa at the start of the experiments; (iii) blockade of cyclooxygenase activity by 3 microM indomethacin. 6. In conclusion, the present experiments have demonstrated that activation of beta-adrenoceptors localized in the mucosa mediates inhibition of [3H]-acetylcholine release from the neuroeffector junctions of the pulmonary, parasympathetic nerves most probably by the liberation of inhibitory prostaglandins from the airway mucosa. The adrenoceptor subtype involved differs in rat (beta 1 subtype) and guinea pig (beta 2 subtype) airways.

Effects of two beta 3-adrenoceptor agonists, SR 58611A and BRL 37344, and of salbutamol on cholinergic and NANC neural contraction in guinea-pig main bronchi in vitro

1. The aim of the present study was to investigate the type of adrenoceptor which modulates constriction of the guinea-pig isolated main bronchus in response to electrical field stimulation (EFS). Drugs used were salbutamol and two agonists reportedly selective for the putative beta 3-adrenoceptor: BRL 37344 and SR 58611A. 2. At basal tone, all three drugs induced relaxation, however, SR 58611A and BRL 37344 (10(-9) to 10(-6) M) relaxed guinea-pig isolated main bronchus more weakly than salbutamol (10(-9) to 10(-6) M). The effects observed at 10(-6) M were 43% +/- 9%, 63% +/- 4% and 98% +/- 1% of the maximal effect induced by theophylline (3 x 10(-3) M) for SR 58611A, BRL 37344 and salbutamol, respectively. 3. SR 58611A and BRL 37344 (10(-8) to 10(-6) M) did not significantly modify the cholinergic component of the response to EFS, but caused a concentration-dependent reduction of the nonadrenergic non-cholinergic (NANC) excitatory component (41.8% +/- 10.1% and 56.8% +/- 7.4% respectively at 10(-6) M, n = 6-7). Salbutamol (10(-9) to 10(-7) M) strongly inhibited both components, with 91.1% +/- 4.2% of inhibition for the NANC contraction and 62.0% +/- 5.2% of inhibition for the cholinergic contraction (10(-7) M, n = 7). 4. Whereas the inhibitory effects of salbutamol were strongly inhibited by both propranolol (10(-6) M) and ICI 118,551 (10(-6) M), those of BRL 37344 were only slightly, albeit significantly reduced by both propranolol and ICI 118,551, and those of SR 58611A were unaffected by treatment with either beta-adrenoceptor antagonist. An alpha2-adrenoceptor antagonist, yohimbine, did not influence the inhibitory effects of any of the beta-adrenoceptor agonists tested.5. Concentration-response curves to acetylcholine (10-8 to 10-3 M), [Nle10]NKA(4-10) (10-10 to10-6 M) and substance P (10- to 3 x 10-6 M) were also significantly shifted to the right by salbutamol(10-6 M), whereas SR58611A and BRL37344 (10-6 M) had no effect.6. These results suggest that the stimulation of putative beta 3-adrenoceptors exerts a specific prejunctional inhibitory action on NANC excitatory response induced by EFS of the isolated main bronchus of the guinea-pig. They also suggest that a beta2-adrenoceptor agonistic component may be involved in the effects of BRL 37344.

Neuronal control of airways smooth muscle

Sensory afferent nerves relay impulses from the airways to the central nervous system so that appropriate changes in bronchomotor tone and breathing patterns may occur. The dominant efferent control of airways smooth muscle is exerted via bronchoconstrictor parasympathetic cholinergic nerves. In some species this is opposed by bronchodilator sympathetic noradrenergic nerves. In addition, there exist both excitatory bronchoconstrictor and inhibitory bronchodilator non-adrenergic, non-cholinergic pathways. This review examines the role of the different branches of the autonomic nervous system in the control of airways smooth muscle tone with particular reference to modulation of these branches and the interactions which may exist between them.

Beta-adrenoceptors on smooth muscle, nerves and inflammatory cells

Although the bronchodilator action of beta 2-adrenoceptor agonists in asthma is largely due to relaxation of airway smooth muscle, these agents have other effects which may contribute to their anti-asthma action. Human airway smooth muscle contains only beta 2-receptors which, when stimulated, stimulate a rise in intracellular cAMP and activation of PKA (protein kinase A), which in turn phosphorylates several cellular proteins, resulting in relaxation. However, beta-agonists also influence membrane K+ channels and induce smooth muscle relaxation without a rise in cAMP, and this mechanism appears to be the major feature of bronchodilatation in asthma. There is also evidence that beta-agonists may modulate neurotransmission in airways via prejunctional receptors on airway nerves, both sensory and motor. Blockade of prejunctional beta 2-receptors in asthma patients may lead to marked rise in acetylcholine release, with severe bronchoconstriction. Although beta-agonists have little or no effect on the chronic inflammatory response which underlies chronic airway hyper-responsiveness, they do inhibit the release of histamine from mast cells in vitro. The presence of beta-receptors has also been detected not only on mast cells but also on eosinophils, macrophages, lymphocytes and neutrophils, but beta-agonists have little or no inhibitory action on the activities of all these cells due to rapid tachyphylaxis.

Excitatory role of axon reflex in bradykinin-induced contraction of guinea pig tracheal smooth muscle

To elucidate the role of the axon reflex in the airways, we studied the effects of atropine (10(-6) M) and tetrodotoxin (10(-7) M) on the bradykinin-induced contraction of guinea pig tracheal smooth muscle, with or without pretreatment of the animals with capsaicin. The concentration-response curves to bradykinin (10(-9) to 10(-5) M) were measured in the presence of both indomethacin and propranolol. In the guinea pigs not given capsaicin pretreatment, baseline tension values did not differ before versus after the application of atropine or tetrodotoxin. Tetrodotoxin reduced the bradykinin-induced contraction significantly, but atropine did not change the contraction induced by bradykinin. These observations indicate that bradykinin-induced contraction is potentiated by a neurally mediated action, but that is not mediated by acetylcholine released from the efferent vagal nerve terminals. The contractile response to bradykinin was significantly reduced in the animals treated with capsaicin as compared with those administered vehicle only. Furthermore, in the animals treated with capsaicin, tetrodotoxin did not affect the response to bradykinin. These observations indicate that bradykinin-induced airway smooth muscle contraction is mediated in part by tachykinins released from C-fiber endings, presumably via an axon reflex.

Airway epithelial cells regulate membrane potential, neurotransmission and muscle tone of the dog airway smooth muscle

1. The effects of epithelial cells were investigated on resting membrane potential and neuro-effector transmission in smooth muscle cells of the dog tracheal and bronchiolar tissues. 2. The mean value of the resting membrane potential of the epithelium-intact bronchiolar smooth muscle cells of the dog was--70.0 +/- 1.1 mV (+/- S.D., n = 40) and mechanical denudation of the epithelial layer depolarized the membrane to -57.0 +/- 2.5 mV (+/- S.D., n = 40). Application of isolated and dispersed epithelial cells (greater than 2 x 10(5) cells/ml) to the perfusing solution repolarized the membrane of epithelium-denuded bronchiolar smooth muscle cells to -67.0 +/- 2.7 mV (+/- S.D., n = 20). The mean resting membrane potential of the mucosa-free tracheal smooth muscle cells was -59.1 +/- 1.4 mV (+/- S.D., n = 50), and application of isolated and dispersed cells (greater than 2 x 10(5) cells/ml) hyperpolarized the membrane to -67.2 +/- 1.8 mV (+/- S.D., n = 50). These repolarizing actions were not modified by indomethacin (10(-5) M). 3. In the epithelium-denuded bronchioles, ACh (greater than 10(-9) M) dose-dependently depolarized the smooth muscle cells, while in the epithelium-intact bronchioles, ACh (10(-11) - 10(-8) M) did not affect the resting membrane potential. At a concentration of 10(-7) M, ACh significantly depolarized the membrane. 4. Electrical field stimulation (EFS; 50 microseconds in duration and about 10-20 V in strength) applied to ring preparations of the bronchioles evoked twitch-like contractions (hereafter referred as twitch contraction), and size of the twitch contractions gradually and continuously decreased in the presence or absence of indomethacin (10(-5) M) and guanethidine (10(-6) M). When similar experiments were performed using epithelium-denuded bronchiolar ring preparations, in no case was there a prominent reduction in the amplitude of the twitch contractions in the presence of indomethacin and guanethidine. 5. The decremental response of the twitch contraction observed in the epithelium-intact bronchioles was overcome by application of the leukotriene synthesis inhibitor AA861 (10(-6) M) and the leukotriene antagonist ONO1078 (10(-5) M). 6. Leukotrienes C4 and D4 (LTC4 and LTD4, greater than 10(-8) M) evoked muscle contraction with a steady increase in muscle tone, up to a certain level. However, at 10(-9) M, LTC4 increased and LTD4 decreased the amplitude of the twitch contractions evoked by EFS in the epithelium-intact bronchioles.(ABSTRACT TRUNCATED AT 400 WORDS)

The role of cyclic AMP in non-adrenergic non-cholinergic contraction in guinea-pig bronchi

1. We investigated the role of adenosine 3':5'-cyclic monophosphate (cyclic AMP) in non-adrenergic non-cholinergic (NANC) contraction in guinea-pig bronchial strips. 2. Forskolin (3 nM to 1 microM) reduced NANC contraction induced by electrical field stimulation (EFS) in a concentration-dependent fashion (-log EC50 was 7.22 +/- 0.12 M and maximum inhibition was 100 +/- 0.01%). However, forskolin (less than 1 microM) did not alter the contraction induced by substance P (SP, 1 microM). 3. Dibutyryl cyclic AMP (1 mM) also reduced NANC contractions induced by EFS (100 +/- 0.01%) without significant effect on SP (1 microM)-induced contractions. In contrast, dibutyryl cyclic GMP (1 mM) was without effect against either NANC or SP-induced contractions. 4. Both the beta 2-adrenoceptor agonist, procaterol (0.1 nM to 3 nM) and theophylline (100 nM to 1 mM) concentration-dependently reduced EFS-induced NANC contractions without significant effect on SP (1 microM)-induced contractions. 5. In contrast to forskolin, procaterol and theophylline, both sodium nitroprusside and cromakalim inhibited the EFS-induced contractions only at those concentrations that similarly reduced the contractions induced by SP (1 microM). 6. These results suggest that cyclic AMP may mediate pre-junctional inhibition of NANC contractions in guinea-pig bronchi.

Cromakalim and K+ channels in canine trachealis

The effects of cromakalim and glibenclamide on membrane properties and responses to acetylcholine of canine trachea were studied in the double sucrose gap to evaluate the presence and function of ATP-sensitive K+ channels. Cromakalim produced a concentration-dependent hyperpolarization of muscle membrane potential which at maximum brought the membrane potential near the potassium equilibrium potential. Current clamping by hyperpolarizing current to this equilibrium potential abolished the hyperpolarization but not the membrane resistance decrease to cromakalim. Glibenclamide had no effect on resting membrane properties but reduced or abolished effects of cromakalim. Another K+ channel antagonist, tetraethylammonium at 20 mM, also reduced the effects of cromakalim, but 4-aminopyridine (5 mM), Ba2+ (1 mM), and apamin (10(-6) M) had no antagonistic effect. The EJP produced on stimulation of cholinergic nerves sometimes increased just after cromakalim-induced hyperpolarization, but within 5-10 min as membrane resistance dramatically fell it was reduced, as was the depolarization to infused acetylcholine. Initially the reduction in EJP amplitude could be partially overcome by applying hyperpolarizing currents or by applying a second field stimulation; later the EJP was reduced further and was unaffected by these procedures. Even when depolarization to acetylcholine was markedly reduced, the contraction was not. Glibenclamide had no effects alone but antagonized all the effects of cromakalim. These results suggest that ATP-sensitive cromakalim activated K+ channels are present in canine trachea but are usually closed during resting conditions under our experimental conditions. When they are opened by cromakalim, they hyperpolarize to near EK, markedly decrease membrane resistance and reduce the depolarization response to acetylcholine, probably by short circuiting the acetylcholine-induced current.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of cholinergic neurotransmission by the peptide VIP, VIP antiserum and VIP antagonists in dog and cat trachea

1. Comparative studies on the effects of vasoactive intestinal polypeptide (VIP), commercially available VIP antiserum or VIP antagonists [Ac-Tyr1, D-Phe2]-GRF(1-29)-NH2 and [4-Cl-D-Phe6, Leu17]-VIP on excitatory neuroeffector transmission in the dog and cat trachea were performed with microelectrode, double sucrose-gap, and tension recording methods. 2. VIP (10(-11)-10(-9) M) had no effect on the resting membrane potential or on the input resistance of the smooth muscle cells of dog and cat trachea. However, with increased concentrations (greater than 10(-8) M) VIP hyperpolarized the membrane and decreased the input resistance of the membrane in both tissues. 3. VIP (10(-10)-10(-7) M) dose-dependently reduced the amplitude of the contractions evoked through the nervous structure excited by field stimulation in the combined presence of indomethacin (10(-5) M) and guanethidine (10(-6) M) in the dog, and in the presence of guanethidine (10(-6) M) in cat trachea. In parallel with actions on twitch contractions, VIP (10(-11)-10(-7) M) reduced the amplitude of the excitatory junction potentials (EJPs) evoked through the nervous structure excited by single pulse field stimulation in both tissues. 4. VIP (10(-9) M) had no effect on the post-junctional response of smooth muscle cells to exogenous acetylcholine (ACh) (10(-9)-10(-5) M). 5. During repetitive field stimulation at the stimulus frequency of 0.033-0.1 Hz, the amplitude of the EJPs was gradually reduced, and VIP (10(-9) M) enhanced this depression phenomenon in the dog and cat trachea. 6. EJPs also showed summation when repetitive field stimulation was applied at high frequency (20 Hz) in the dog trachea. The slope of the relationship between the relative amplitude of the EJP and number of stimuli at 20 Hz was 2.2 +/- 0.4 mV/stimulation (n = 4) in the dog trachea. However, in the cat trachea, summation of EJPs was not prominent, giving a mean slope of 0.6 +/- 0.2 mV/stimulation (n = 6) measured by the microelectrode method. VIP (10(-9) M) shifted downward the relationship between the relative amplitude of the EJP and the number of stimuli at 20 Hz in both tissues. 7. Overnight incubation with VIP antiserum (10(-6) g/ml) had little effect on the depression of the EJP in the dog and cat trachea, or the summation of the EJP observed in the dog trachea.(ABSTRACT TRUNCATED AT 400 WORDS)

Pre-junctional inhibitory action of prostaglandin E2 on excitatory neuro-effector transmission in the human bronchus

The effects of prostaglandin E2 (PGE2) and indomethacin on excitatory neuro-effector transmission in the human bronchus were investigated by tension recording and microelectrode methods. PGE2 (10(-10)-10(-9)M) suppressed the amplitude of twitch contractions and excitatory junction potentials (e.j.ps) evoked by field stimulation at a steady level of basal tension obtained by the combined application of indomethacin (10(-5) M) and FPL55712 (10(-6) M). In doses over 10(-8)M, PGE2 reduced the muscle tone and dose-dependently suppressed the amplitude of twitch contractions. Indomethacin (10(-5) or 5 x 10(-5) M) reduced the muscle tone and enhanced the amplitude of twitch contractions and e.j.ps evoked by field stimulation in the presence of FPL55712. PGE2 (10(-9) M) had no effect on the post-junctional response of smooth muscle cells to exogenously applied acetylcholine (ACh) (4 x 10(-7) M). However, indomethacin (10(-5) M) significantly enhanced the ACh-induced contraction of the human bronchus. These results indicate that PGE2 in low concentrations has a pre-junctional action to inhibit excitatory neuro-effector transmission in addition to a post-junctional action, presumably by suppressing transmitter release from the vagus nerve terminals in the human bronchial tissues.